A new mechanism for negative differential resistance (NDR) has been discovered and involves a single molecule in the junction of a scanning tunneling microscope. The NDR was observed at voltages corresponding to the vibrational energies of single pyrrolidine molecules on the (001) surface of copper at 9 K. This vibrationally mediated NDR contrasts with the previously known mechanisms which all involve the electronic states of the conducting media.

Nature has evolved dynamic, non-equilibrium mechanisms for assembling hierarchical complexes of nanomaterials. A critical element to many of these assembly mechanisms involves the active and directed transport of materials by biomolecular motor proteins such as kinesin. In the present work, nanocrystal quantum dots (nQDs) were assembled and organized using microtubule (MT) filaments as nanoscale scaffolds. nQD density and localization were systematically evaluated by varying the concentration and distribution of functional groups within the MT structure. Confining nQD attachment to a central region within the MT enabled unaffected interaction with kinesin necessary to support active transport of nQD−MT composites. This active transport system will be further refined to control the optical properties of a surface by regulating the collective organization of nQD−MT composites.

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The dynamics and chemistry of individual ethylene molecules adsorbed on the Ni(110) surface at 13 K have been studied with a variable temperature scanning tunneling microscope (STM). By applying a voltage pulse to a single ethylene molecule, the tunneling electrons cause the molecule to reversibly hop away from and back under the tip. The five ethylene isotopes hop at different rates. A larger voltage pulse (1.1−1.5 V) induces dehydrogenation, and a strong primary isotope effect is observed. By using inelastic electron tunneling spectroscopy (IETS) with the STM, we identified the dehydrogenated products from the characteristic vibrational energies as acetylene. This identification is further supported with STM-IETS on single acetylene molecules adsorbed directly from the gas phase. Two different types of acetylene exist on the surface. They can be distinguished in the regular STM image, and they exhibit shifted vibrational peaks. Applying another voltage pulse (1.0 −4.8 V) further dehydrogenates acetylene to carbon atoms.